Digitally controlled oscillators (DCO) control the output frequency using digital control. Some digitally controlled oscillators output a signal at a frequency Fout according to output frequency control data that is input as a digital value. The output frequency control data which is input to such a digitally controlled oscillator is referred to as a “tuning word” (hereafter also referred to as the “TW”).
FIG. 1 is a block diagram illustrating an example of the configuration of a conventional digitally controlled oscillator. As illustrated in FIG. 1, a conventional digitally controlled oscillator 90 has a crystal oscillator (X'tal Oscillator or XO) 91, an adder 92, an amplitude value acquisition unit 93, an amplitude value table 94, a digital to analog converter (DAC) 95, and a low-pass filter (LPF) 96.
The crystal oscillator 91 supplies an operation clock of frequency FS to the adder 92 and the DAC 95.
According to the operation clock of frequency FS, the adder 92 having an N-bit length cyclically adds TWs which is input to the adder 92. That is, the adder 92 cyclically adds TWs with timing TS that is equivalent to the frequency FS. The addition value outputted from the adder 92 is input back to the adder 92 and as well input to the amplitude value acquisition unit 93. The TW value is a digital value, so that the addition value outputted from the adder 92 is also a digital value. For example, the addition value outputted from the adder 92 increases with time as illustrated in FIG. 2. FIG. 2 is a diagram illustrating an example of output from the adder.
The amplitude value acquisition unit 93 acquires, from the amplitude value table 94, an amplitude value corresponding to an addition value input from the adder 92. In the amplitude value table 94, held are amplitude values of a sine wave for one period, each of the amplitude values being associated with each addition value from the minimum to the maximum addition values obtained by the adder 92. The amplitude value acquisition unit 93 refers to the amplitude value table 94 according to an addition value which is input to the amplitude value acquisition unit 93 and acquires an amplitude value corresponding to the addition value to output the amplitude value to the DAC 95. This amplitude value is a digital value.
The DAC 95 converts the digital amplitude value into an analog amplitude value according to the operation clock of the frequency FS and then outputs the converted analog amplitude value to the LPF 96. That is, the DAC 95 converts the amplitude value from digital to analog with the timing TS that is equivalent to the frequency FS. Since each amplitude value of a sine wave for one period is held in the amplitude value table 94, the DAC 95 outputs a stepped sine wave as illustrated in FIG. 3. FIG. 3 is a diagram illustrating an example of output from the DAC. The one period Tout of the sine wave is the reciprocal of the output frequency Fout of the digitally controlled oscillator 90.
The LPF 96 filters out high-frequency components outputted by the DAC 95, that is, the high-frequency components of the stepped sine wave, and then outputs a sine wave with the high-frequency components filtered out. Thus, as illustrated in FIG. 4, the LPF 96 outputs a sine wave that has a period Tout (Tout=1/Fout). FIG. 4 is a diagram illustrating an example of output from the LPF. The output of the LPF 96 serves as the output from the digitally controlled oscillator 90. That is, the digitally controlled oscillator 90 outputs a signal at a frequency of Fout.
Here, the output frequency Fout from the digitally controlled oscillator 90 is defined by the TW value “TW,” the bit length “N” of the adder 92, and the operation clock frequency “FS,” and expressed by Equation (1) below.
                              F          out                =                              TW                          2              N                                ·                                    F              S                        ⁡                          [              Hz              ]                                                          (        1        )            
Related-art examples are described, for example, in Japanese Laid-open Patent Publication No. 05-336181 and Japanese Laid-open Patent Publication No. 2012-060395.
FIGS. 5 and 6 illustrate the phase noise characteristics of the conventional digitally controlled oscillator 90 mentioned above. FIGS. 5 and 6 each are a diagram illustrating an example of an actually measured phase noise characteristic. FIG. 5 is an example of an actually measured phase noise characteristic when the TW value is such that TW=268,435,456, and Fout=10.886392 MHz. FIG. 6 is an example of an actually measured phase noise characteristic when the TW value is such that TW=268,432,771, and Fout=10.886288 MHz. In FIGS. 5 and 6, the horizontal axis represents the offset frequency corresponding to the output frequency Fout and the vertical axis represents the strength of phase noise. In FIG. 5 with TW=268,435,456, relatively low spurious components about 10 to 20 in number are observed. In contrast to this, in FIG. 6 with TW=268,432,771, a number of relatively high spurious components are observed when compared with FIG. 5. Particularly, in FIG. 6, higher spurious components are found over the entire region near an offset frequency of 800 Hz or higher when compared with FIG. 5. As described above, the conventional digitally controlled oscillator 90 may have considerable increased spurious components for a particular TW, for example, for TW=268,432,771. That is, in the conventional digitally controlled oscillator 90 is found such a phenomenon that for a particular TW, the phase noise characteristic considerably deteriorates. Thus, for example, a data communication apparatus that employs the output from the conventional digitally controlled oscillator 90 as a clock will deteriorate in jitter characteristic at the time of data transmission or data reproduction at a particular output frequency of the digitally controlled oscillator 90.